Formation testers are used at various times in drilling and/or producing a well to create gradient plots where recorded formation pressures are plotted against formation depth and used to determine in situ fluid densities. This information may then be used to identify the type of fluid in the formation, such as oil, gas, and/or water. Additionally, more than one type of fluid, i.e., oil, water, and/or gas, may be located within a particular zone of interest. When there are two or more fluids present in a continuous zone and two fluid gradients can be identified then the intersection of these gradients is assumed to be the fluid contact. If one of the fluids in a formation is determined to be water, the operator may decide to perforate the well above the water level. Therefore, the depth of the changes from one fluid to another, or fluid contacts, within a formation zone are analyzed to determine the overall design of the well or estimating reserves of producible hydrocarbons within an already producing formation. The pressure gradient plots may also be used to determine whether zones within a formation are fluidly connected.
Developing a gradient plot for analysis involves operating a formation tester within a well bore to measure formation pressures at various planned locations at different depths. Choosing how many locations and at what depths to perform the pressure tests may affect the overall results of the gradient analysis and the acceptable reliability of the analysis. Theoretically, the more pressure tests performed allows more data points to be measured, resulting in a more accurate gradient plot. However, more pressure tests take more time and thus may raise the overall expenses of drilling a well. Additional reliability variables include the accuracy of the measurement instruments and the methods of data analysis once obtained The properties of a single type of fluid may also change at different depths within the same formation due to biodegradation or other causes The reliability of the analysis and the economics of obtaining the measurement data are balanced in generating a pressure gradient.